In any manufacturing involving the use or movement of particulate materials, some breakdown or attrition of the particles is inevitable and has been reported in a wide range of processes and industries. For example, it has significance for those applications where it is desirable for particles to remain in a process almost indefinitely. The effects of attrition can be loss of product by removal of undersize particles, the need for recycling lost product, and the requirement for additional filtration. Another effect can be to limit the useful life of catalyst or enzyme particles.
Many products in the pharmaceutical industry, for example, are agglomerated granules which can suffer attrition during processing and also, if bulk packed, during shipment and use. Dust release into the atmosphere may be a hazard, but its release is also undesirable because of the high value of many of the products.
Attrition has a number of different effects, the relative importance of which is dependent upon the commercial or technical application. Properties of particulate materials change as a result of attrition. Loss of material occurs due to the change of particle sizes to smaller ones which are unacceptable to the particular process and which are removed from the process by accident or design in cyclones, filters, or precipitators. Wear of contaminant systems results from the impact of particles with the walls of the container or duct, and contamination of the process particles by debris from wear of the containment system may be significant in some applications. Even an explosion can be caused if a build-up of fine material is allowed to occur.
Several conventional methods for testing the mechanical strength of industrial catalysts which are used in fluid beds are reported in C. R. Bemrose and J. Bridgwater, A Review of Attrition and Attrition Test Methods, Powder Technology, 49 (1987) 97-126. One of the vibration tests discussed therein used a container enclosing a granular charcoal bed which was vibrated at 60 Hz with an acceleration of 5 g. Air was blown into the top of the container and dust formed by the attrition of the granular charcoal passed through the perforated base of the container to be collected on a glass fiber filter paper. As a result, the impact strength of the entire bed was tested and not individual particles.
Another vibration test is reported in T. P. Ponomareva, S. I Kontorovich, and E. D. Shchukin, Attrition Of Spherical Cracking Catalysts in the Presence Of Powdered Lubricants, Kinetics and Catalysis, 21 (1980) 505-510 for measuring the wear between catalyst particles treated with a lubricant powder. The test used a specially constructed cylindrical drum undergoing vertical vibrational movement imposed by a vibro-saw at a frequency of 50 Hz and an amplitude of 6 mm. Only the amount of wear between the catalyst particles themselves could be measured by removing the abrasion products through sieves located in the drum.
These methods have proven unsuitable for characterizing the individual particle since these methods vibrate an entire bed within a closed vessel. They measure the particle to vessel-wall interaction when the attrition and fragmentation of the particles is primarily caused by the particles rubbing each other. The impact velocity of the collisions encountered by the particle bed is also poorly defined by these methods and may be inaccurate due to some collisions encountering drag forces within the bed. Furthermore, some of these methods only measure spherical particles and are poorly adapted to accurately characterize aspherical or non-uniform shaped particles.
Thus, a need exists for a method and device for assessing attrition and fragmentation characteristics of particles during handling. A tool is needed to help develop particles which are individually strong since the mechanical stability of the particle is of primary importance in many industrial fields such as enzyme formulation technology. Characterizing the impact strength of individual particles, as opposed to entire particle bed, provides information that can be used to develop particles with enhanced attrition strength.